Question

A two winding transformer is rated at 60 kVA, 240/1200 V, at 60 Hz with efficiency of 0.96. If the same transformer is used as as a 1440/1200 V step down auto transformer in a power system, the kVA rating of the auto transformer will increase by ______ times.

• A. 6

Answer 1

The correct option is A 6
Given two winding transformer: 60 kVA, 240/1200 V

$\text{Primary rated current,}{I}_1=\frac{60000}{240}=250A$

$\text{Secondary rated current,}{I}_2=\frac{60000}{1200}=50A$

Now above transformer is connected as step down auto transformer as showwn below.




$\text{Auto transformer secondary current}$

$\text{= 250 + 50 = 300 A}$

$\text{For rated current, auto transformer rating}$

$\text{S = (1200)}\times \text{(300)}\times {10}^{-3}$

$\therefore \frac{\text{Rating of auto transformer}}{\text{Rating of two winding transformer}}=\frac{360}{60}=6$